This application claims priority to EP/01108173.4, filed Mar. 30, 2001 under the European Patent Convention and which is incorporated by reference herein in its entirety.
1. Field of the Invention
The invention relates to a gas turbine having a number of moving blades in each case combined to form moving-blade rows and arranged on a turbine shaft and having a number of guide blades in each case combined to form guide-blade rows and connected to a turbine casing.
2. Background of the Invention
Gas turbines are used in many fields for driving generators or driven machines. In the process, the energy content of a fuel is used for producing a rotational movement of a turbine shaft. To this end, the fuel is burned in a combustion chamber, in the course of which air compressed by a compressor is supplied. In this case, the working medium which is produced in the combustion chamber by the combustion of the fuel and is under high pressure and high temperature is directed via a turbine unit connected downstream of the combustion chambers, where it expands to perform work.
In this case, to produce the rotational movement of the turbine shaft, a number of moving blades normally combined in blade groups or blade rows are arranged on the turbine shaft and drive the latter via an impulse transfer from the working medium. In addition, in order to direct the working medium in the turbine unit, guide-blade rows connected to the turbine casing are normally arranged between adjacent moving-blade rows.
In addition to the output achievable, an especially high efficiency is normally a design aim when designing such gas turbines. In this case, an increase in the efficiency, for thermodynamic reasons, can in principle be achieved by increasing the outlet temperature with which the working medium flows out of the combustion chamber and into the turbine unit. Therefore temperatures of about 1200xc2x0 C. to 1300xc2x0 C. are aimed at for such gas turbines and are also achieved. At such high temperatures of the working medium, however, the components exposed to the working medium are subjected to high thermal loads. In order to nonetheless ensure a comparatively long service life of the affected components with high reliability, cooling of the affected components, in particular of moving and/or guide blades of the turbine unit, is normally provided. The turbine blades are therefore normally designed to be coolable, in which case, in particular, effective and reliable cooling of the first blade rows as viewed in the direction of flow of the working medium is to be ensured. For the cooling, the respective turbine blade in this case normally has a cooling-medium passage which is integrated in the blade body or the blade profile and from which a cooling medium can be specifically directed in particular to the thermally loaded zones of the turbine blade.
The cooling medium used in this case is normally cooling air. This cooling air is normally fed to the respective turbine blade in the manner of an open cooling system via an integrated cooling-medium passage. After discharging from the turbine blade, the cooling air is mixed with the working medium directed in the turbine unit. However, the design output of a gas turbine cooled in this way is limited, in particular since, in view of the limited mechanical loading capacity of individual components of the gas turbine, a further increase in output can normally only be achieved by an augmented supply of fuel. This in turn results in a comparatively increased demand for cooling medium for cooling the turbine blades, and this demand in turn means losses in the available compressor mass flow. These losses in turn can only be tolerated to a limited extent.
The object of the invention is therefore to specify a gas turbine of the abovementioned type which, with reliable cooling of the turbine blades, is suitable for an especially high design output.
This object is achieved according to the invention by the integrated cooling-medium passage of a guide blade being split up into a first and a second partial-flow passage in such a way that a first partial flow of the cooling medium flowing in the first partial-flow passage is largely used for cooling this guide blade, and that the second partial flow of the cooling medium flowing in the second partial-flow passage is passed on more or less free of losses.
In this case, the invention is based on the idea that an especially high design output can be achieved at a high efficiency of the gas turbine by losses being kept especially low in the available compressor mass flow. In order to keep these losses especially low, the gas turbine should therefore be designed for an especially low demand for cooling medium, in particular cooling air. This can be achieved by virtue of the fact that, while abandoning the concept of an open cooling system, a closed cooling system is provided in which the cooling medium is supplied to the combustion process after flowing through the turbine blades to be cooled. However, a pressure loss possibly occurs in the cooling-medium line in a cooling system closed in such a way, and this pressure loss in principle rules out the possibility of feeding the cooling medium, flowing off from the turbine blades, into the combustion chamber of the gas turbine.
In order to firstly keep this pressure loss especially low and thus even make possible a cooling system, which is in principle closed, for the turbine blades, provision is made to split up the cooling medium inside the guide blade, one partial flow of the cooling medium cooling the respective guide blade, and the second portion of the cooling medium being directed through the guide blade virtually free of losses. Thus, on the one hand, sufficient cooling medium can be admitted to the guide blade itself, and, on the other hand, further guide blades to be cooled are likewise provided with sufficient cooling medium virtually free of pressure losses. This splitting-up of the cooling medium into two partial flows, with regard to the pressure of the cooling medium, means parallel connection of the cooling systems of the actual guide blades and of the guide blades of the following turbine stage. Abandoning the concept of sequential cooling-medium guidance through the guide blades of adjacent turbine stages and through the guide ring connecting the latter enables the cooling-medium passages to be directed, and this reduces the pressure loss in the cooling medium. In addition, the guidance of the cooling medium is simplified, since the cooling-medium passages are split up in the guide blade. Furthermore, the flow paths relevant to pressure loss can be kept short, and the pressure loss in the cooling medium can accordingly also be kept small, which brings about a reduction in the cooling-medium quantity.
In this case, provision is preferably made for a sufficient quantity of cooling medium to be specifically admitted to the blade profile of the respective guide blade. This is because it is precisely the blade profile of the guide blades forming the first guide-blade row that is subjected to especially high thermal loading, so that it is precisely here that the demand for sufficient cooling is especially high. Cooling medium can therefore be specifically admitted to this region, it also being possible to maintain especially short flow paths for the corresponding cooling-medium flow. Even if a comparatively large quantity of cooling medium is admitted to the blade profile, the pressure loss can thus be kept limited, so that it is possible for the cooling medium flowing off from the blade profile to be fed back into the combustion process of the gas turbine.
In a further advantageous configuration, the cooling medium directed in the second partial-flow passage is provided for the cooling in the guide blade of the second guide-blade row. To this end, the second partial-flow passage is advantageously connected on the outlet side to a cooling-passage system if it directs the cooling medium first of all largely free of losses to the guide blade of the second guide-blade row. The cooling medium flowing through the second partial-flow passage is now sufficiently available for cooling the guide blade of the second guide-blade row. The xe2x80x9cspentxe2x80x9d cooling medium of the guide blades of the first and second guide-blade rows is then fed via a further cooling-medium passage to the combustion chamber of the gas turbine. The essential pressure drop in the cooling medium is thus kept very low, since the second partial flow of the cooling medium only flows through the guide blades of the second guide-blade row.
It is precisely through this second partial-flow passage that the guide blades forming the second guide-blade row can be fed with a quantity of cooling medium which meets the demand. In particular on account of the lower temperature, compared with the first guide-blade row, to which the second guide-blade row is exposed, the quantity of cooling medium provided for cooling the guide blades forming the second guide-blade row can be proportioned so as to be comparatively small, so that the pressure loss is only limited despite the comparatively long flow path of the cooling medium in the second partial-flow passage.
In an especially advantageous configuration, the cooling-medium compressor connected upstream is to be dimensioned in such a way that the pressure of the xe2x80x9cspentxe2x80x9d cooling medium, before being fed back into a flow space surrounding the combustion chamber, is virtually identical to the pressure which prevails in the flow space. Such an arrangement ensures that the cooling medium provided for the guide blades has a sufficiently high pressure level, so that at least most of the cooling medium required can be fed back into the combustion process of the gas turbine. It is precisely due to the feedback of the cooling air required into the combustion process that an especially high power output and/or an especially low degree of emissions can be achieved.
The advantages achieved with the invention consist in particular in the fact that the branching of the flow path for the cooling air in the guide blades forming the first guide-blade row enables the cooling air, with a comparatively low pressure loss, to be specifically admitted to the components to be cooled. The pressure loss, which thus occurs only to a limited extent, can be compensated for in particular by the cooling-medium compressor connected upstream on the cooling-medium side, so that, at an invariably high efficiency, recovery of the cooling air for the combustion process, in such a way as to increase output and reduce emissions, is made possible in the manner of a closed cooling system for the respective guide blades. In this case, the cooling-air flow, in such a way as to meet the requirements, is split up in the guide blades forming the first guide-blade row, the first partial flow, with a tolerable pressure loss, being provided for cooling the guide-blade profile, and the second partial flow flowing through this guide blade essentially free of losses. Consequently, the second partial flow, with a tolerable pressure loss overall, can still be used for cooling the guide blades forming the second guide-blade row. The matching in general, with regard to the pressure conditions prevailing in the cooling medium, between the cooling-medium compressor connected upstream and the arrangement of the splitting-up of the cooling-medium passages for parallel admission of cooling medium to the guide blades of adjacent turbine stages permits a reduction in the requisite cooling medium while at the same time maintaining the requisite cooling with the use of the closed cooling-medium guidance.